A Single-Chain Antibody protein with specificity for human IgM will be prepared. The SCA protein will be designed from a variable of the anti-human IgM monoclonal antibody, DA4.4. A gene to synthesize this protein will be constructed and the protein expressed in E. coli. The protein will be folded, purified, and assayed for its binding to human IgM. This protein will be used as a reagent for the purification of human IgM monoclonal antibodies, as a diagnostic reagent for antigen specific IgM, and as a therapeutic in extra corporeal blood filtration for diseases, e.g., cancer, and AIDS. The immobilizations necessary for this protein to perform in these reactions will be developed during Phase II.